1. Field of the Invention
The present invention relates to the control of gaseous effluents produced by processes for the production of aluminum by igneous electrolysis. It relates more particularly to the collection of toxic gaseous compounds, in particular fluoride compounds, liberated by spent anodes and by the bath crust after they have been removed from the electrolytic pots.
2. Discussion of the Background
Metallic aluminum is produced industrially by igneous electrolysis, that is by the electrolysis of alumina in a bath of molten cryolite by the well-known Hall-Heroult process. However, the electrolysis reaction, the secondary reactions and the high operating temperatures lead to the production of gaseous effluents which mainly contain carbon monoxide and dioxide and fluoride compounds.
The emission of these effluents into the atmosphere is strictly controlled and regulated not only with regard to the electrolysis room environment to safeguard the personnel working in the vicinity of the pots but also with regard to atmospheric pollution. Pollution regulations in many countries impose limits on the quantities of effluents emitted into the atmosphere.
There are present solutions which allow these effluents to be extracted, recovered and treated reliably and satisfactorily. A widely adopted solution involves equipping these electrolytic pots with an effluent collection device. This device covers the electrolytic pots and comprises a hooding device and a system for the suction and chemical treatment of the effluents. Known processes for treating effluents include, in particular, the recovery of fluoride gases by reaction with alumina. The hooding device includes access devices such as hoods and a tapping door which allow intervention into the pot. The tapping door, which is generally located at one of the ends of the pot on the traffic aisle side, allows a portion of the liquid aluminum produced during electrolysis to be extracted easily.
The hooding device defines a confined suction zone subjected to a vacuum relative to the environment, which allows the effluents to be effectively recovered. Gas collection yields as high as 99% are obtained during continuous running in the latest industrial installations, such that the rates of emission of fluoride gaseous compounds into the atmosphere are much lower than the statutory thresholds.
However, pollution regulations are constantly changing and the standards in force are becoming stricter. In particular, in some aluminum producing countries, the emissions of fluoride compounds, which are at present usually limited to less than 1 kg F (total fluorine) per tonne of aluminum produced (F/tonne Al), are likely to be limited to less than 0.5 kg F/tonne Al in the near future. There is therefore a need for a way of further reducing the atmospheric emissions from factories which produce aluminum by igneous electrolysis in order, in particular, to comply with the strictest standards, but which are satisfactory with regard to production costs.
Known processes for the production of aluminum by igneous electrolysis, which essentially run continuously, involve regular intervention in the electrolytic pots, which makes it necessary to open the hooding device, thus breaking the confinement of the suction zone. This is the case, in particular, during operations for changing spent anodes. The operations for changing spent anodes involve opening removable hoods to provide access to the anodes to be changed, and removing these anodes from the electrolytic pots and therefore from the suction zone of the hooding device of these pots. The crust is thus removed from the pots during changes of anodes either by being carried off by the spent anodes removed from the pots or by being intentionally removed so as to facilitate the positioning of the replacement anodes, among other things. The anodes and the bath crust removed from the pots in this way are very hot, i.e., at a temperature generally higher than 800.degree. C., and consequently have to undergo a cooling phase to ambient temperature. In general, the anodes and the bath crust are then treated so as to recover the constituent materials, usually when they have reached a temperature close to ambient temperature.
The spent anodes and the bath crust removed from the electrolytic pots during changes of anode evolve significant quantities of gaseous compounds during their cooling phase. These gaseous compounds consist mainly of fluoride compounds. The rate of evolution of gaseous compounds is initially very high and then decreases rapidly in the hour following removal from the electrolytic pots, in particular due to the reduction in the temperature. The rates of evolution of fluoride compounds are typically of the order of 10 to 100 g of F per minute, only in the case of the spent anodes.
Consequently, although the suction devices are generally designed such that the opening of the hoods barely affects the effectiveness of suction during the operations of changing spent anodes, the removal of the anodes and of the crust from the suction zone leads to the direct emission into the atmosphere of the toxic gaseous compounds, in particular fluoride compounds, evolved by them during the handling and cooling thereof.
There is therefore a need for significantly reducing the quantity of gaseous compounds liberated directly into the environment by the spent anodes and the bath crust removed from the electrolytic pot during the operations of changing spent anodes and during the cooling of said anodes and crust. However, few means are known for satisfactorily achieving these objectives, in particular with regard to investment costs and process control and, more particularly, with regard to the limit of 0.5 kg F/tonne Al for emission of fluoride compounds.
It has recently been proposed in German application DE 42 21 882 Al to place the spent anodes in a transport and intermediate storage device. This device consists of a container possessing one or more compartments equipped with sealed hoods and in which the spent anodes are deposited. After closure of the hoods, the internal pressure in the container is raised due to heating of the internal atmosphere and accumulation of the gases evolved by the spent anodes. The gases are discharged into the environment by passing through filters which trap the toxic gases. However, this solution necessitates an adequate seal for the container which is at excess pressure relative to the ambient pressure, involving an increase in the production costs. Furthermore, the size of the filters and therefore their effectiveness is limited by the dimensions of the container and by the excess pressures permitted in the container.